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Abu Ishaq Ibrahim Ibn Yahya Al-Zarqali

Al-Zarqali is an eminent Andalusian astronomer of the 11th century who was the foremost astronomer of his time. He excelled in different domains of theoretical and practical astronomy and left works that influenced greatly his followers in the Islamic Andalusian and North-African and Latin astronomical traditions, until Copernicus in the 16th century.

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Abu Ishaq Ibrahim Ibn Yahya Al-Zarqali (Arzachel in Latin), was an Andalusian famous scholar known also as Al-Zarqalluh and Al-Zarqallah. He was born in 1029 and died in 1087. His life corresponded exactly to that dramatic period when the Muslim realm of Spain completely disintegrated, and nearly collapsed, only to be saved in 1089, when the Almoravids of Morocco crossed into Spain, crushed the Christians at Zalaqa, and unified Spain once more. They were followed later by the Almohads, who kept Spain under Muslim rule for another two centuries, until the middle of the 13th century, when Muslim Spain, with the exception of Granada, was lost for good.

Background to Al-Zarqali's life

Before the agitated period corresponding to Al-Zarqali's life, Spain was the light in Europe. It had scientists of unique calibre, the likes of Ibn Firnas, for instance (d. 887), a poet, a mathematician, an astronomer and physicist at the Spanish Ummayad court under three successive rulers, who could decipher even the most incomprehensible hieroglyphics [1]. He also invented spectacles, complex chronometers, and a flying machine [2]. Al-Zahrawi, also known as Albucasis in Latin (936-1013), wrote on medicine, pharmacology, and also on cookery and dietetics, medical chemistry, therapeutics, and above all surgery. Abu Hanifa al-Dinawari (d. 895) left extensive works in botany; his works were brought to us by the German scholar Silberberg [3]. Al-Majriti (d. 1007) was an astronomer, a chemist, and he also wrote treatises on commercial arithmetic and on taxes, using algebraic, geometrical, and arithmetical operations.

Cordova, the capital of Muslim Spain, used to be the jewel of Europe which dazzled visitors from the North [4]. It was "the most civilised city in Europe, the wonder and admiration of the world, a Vienna among Balkan states [5]." Its streets were paved and well lit [6]. One could travel for ten miles by the light of street lamps, and along an uninterrupted series of buildings [7]. It also had around 70 public libraries during the time of Caliph Hakam II, and 900 public baths [8]. Included amongst its libraries were the great mosque libraries which were open for anyone to go and use. And whenever the rulers of Leon, Navarre or Barcelona needed a surgeon, an architect, or a dress maker, they applied to Cordova [9].

Figure 1: View of Toledo, the city of Al-Zarqali, dominated by the famous Alcazar (Source).

This brilliance was pursued under al-Mansur (also known as Ibn abi Amir), who ruled on behalf of the young prince Hisham, from 976 to 1002. Al-Mansur kept the northern Christians under check, mounting two military campaigns every year against them. At home, Al-Mansur extended the great mosque of Cordoba, and built the magnificent al-Zahra City. Under him, Spain was also at the peak of its wealth, visitors stunned by the mountains of riches.

Soon after the death of the great leader al-Mansur, Muslim Spain fell into chaos, the era of the "party kings" (reyes de taifas, muluk at-tawa'if) (1009-1091), when the Peninsula broke into as many as thirty more or less independent rulers, who fought each other [10]. Encouraged by this, Christian princes in North West Spain swept south, conquering one Islamic kingdom after the other, very often using one against the other [11]. Barbastro was taken in 1063 from the Muslims [12]. In 1085, the old Visigoth capital of Toledo fell to Alfonso VI of Castile [13], a victory achieved with the help of al-Mutamid of Seville [14]. Al-Mu'tamid, himself, was soon threatened [15]. In panic, with other kings, he called the Almoravids, and their leader Yusuf Ibn Tashfin, to protect them from the invasion of the Castillan troops. Ibn Tashfin crossed into Spain on three occasions, each time after crushing the Christian armies, he was invited to leave. The third time he was invited in 1090, Ibn Tashfin crossed the straight of Gilbraltar, and this time eliminated the Reyes de taifas, and installed Almoravid rule all over the country [16]. The Almoravids, followed by the Almohads kept Andalusia in Muslim hands until the 13th century. But disunity and infighting led this time to a final disintegration. Cordova fell in 1236, Seville in 1248, and soon followed the other towns and cities, only leaving the Grenada enclave which fell later in 1492.

To understand what the fall of such Muslim towns and cities, and the fall of Islamic power meant for Muslim scholarship, nothing better than the life of Al-Zarqali, who lived in the first period of chaos when the Castillans attacked the disunited Reyes of Taifas, before the Almoravid intervention stopped the upcoming invasion. Like other Muslim scholars who lived during this chaotic period, each time, fleeing from one place to another as the Christians advanced; Al-Zarqali, for instance, fled Toledo his hometown when it was threatened by the Castillans, before it fell in 1085. Many of the leading Muslim Spanish scholars and men of letters, lived through the same experience. Ibn Bassam describes how the incessant invasions of the Castillans forced him to run away from Santarem in Portugal, "the last of the cities of the west," after seeing his lands ravaged and his wealth destroyed, a ruined man with no possessions save his battered sword [17]. Many scholars such as Abu Salt of Denia, and Abu Behr al-Tortuchi of Tortosa left Spain altogether to take refuge in Egypt [18]. Others were still more unfortunate, like the poet Ibn Wahbun, who was killed by the Castillans on the road from Lorca to Murcia in 1087 [19].

Al-Zarqali's life and achievements

Al-Zarqali, Barron Carra de Vaux tells us, was given the surname ‘Al-Nekkach', that is the engraver of metals [20]. According to established tradition, he was a mechanic and metal craftsman, very crafty with his hands [21]. It is as an instrument maker that al-Zarqali entered the services of Cadi Ibn Said of Toledo. He would make delicate instruments needed to continue astronomical observations that begun in 1060, possibly begun by Yahya Ibn Abi Mansur [22]. First Al-Zarqali built instruments for other scholars, but when they realised his great intellect, they became interested in him. As he told them he was man of little learning, having never studied any science, nor even touched a book, they put him to task, and made him study and learn [23]. They put at his disposal books he needed to educate himself [24]. And two years later, in 1062, he became a member of the group, and soon after the director of this very group [25]. Al-Zarqali kept on making instruments for others which they asked for, but now began to invent his own, and even sooner, he began to teach his own masters, to the point, that they soon began to follow him, and calculate exactly as he did [26].

Figure 2: Spanish stamp of Al-Zarqali with astrolabe.

Al-Zarqali constructed the famed clocks of Toledo, which al-Zuhri has described in a Castilian translation, published by J.M. Millas-Vallicrosa [27]. The clocks were in use until 1135, when King Alphonso VI tried to discover how they worked and asked Hamis Ibn Zabara to dismantle them [28]. Once they were taken apart, nobody could reassemble them. They constituted a very precise lunar calendar and were, to some extent, the predecessors of the clocks or planetary calendar devices that became fashionable six centuries later in Europe [29].

Ahmad Thomson has given a vivid account of the intricate working of the clocks. The clocks consisted of two basins, which filled with water or emptied according to the increasing or waning of the moon. At the moment when the new moon appeared on the horizon, water would begin to flow into the basins by means of subterranean pipes, so that there would be at day-break the fourth of a seventh part, and at the end of the day half a seventh part, of the water required to fill the basins. In this proportion the water would continue to flow until seven days and as many nights of the month had elapsed, by which time both basins would be half filled. The same process during the following seven days and nights would make the two basins quite full, at the same time that the moon was at its full. However, on the fifteenth night of the month, when the moon would begin to wane, the basins would also begin to lose every day and night half a seventh part of their water, until by the twenty-first of the month they would be half empty, and when the moon reached her twenty-ninth night not a drop of water would remain in them. It is worthy of remark that, should anyone go to any of the basins when they were not filled, and poured water into them with a view to quicken its filling, the basins would immediately absorb the additional water and retain no more than the just quantity; and, on the contrary, were anyone to try, when they were nearly filled, to extract any or the whole of their water, the moment he raised his hands from the work the basins would pour out sufficient water to fill the vacuum in an instant [30].

This was only one aspect of Al-Zarqali's achievements, for he constructed the most sophisticated and precise astrolabe ever. The astrolabe was a composite astronomical instrument which performed a variety of operations. The most common form, the planispheric astrolabe, had on its front a zodiacal circle and a disc (safîha or azafea, in medieval Castilian) designed for a specific geographical latitude, with a stereographic projection of the equator, the tropics, and the horizon [31]. Which made possible the solution of various problems of spherical astronomy, and to measure the hour of the day [32]. Al-Zarqali invented constructed and wrote al-Safiha al-Zarqaliya (Azafea), a treatise on the universal astrolabe; an instrument out of which a whole literature developed later [33]. It was a stereographic projection for the terrestrial equator and could be used to solve all the problems of spherical astronomy for any latitude [34]. It includes a table of 29 stars with ecliptical coordinates intended to be marked on the instrument [35]. The refined astrolabes were also used for observing the sun's movement [36]. This innovation, which avoided the inconvenience of having to change the safîha for each latitude. The universal plate impacted on subsequent Western science; as illustrated in the Alfonsine Libros de saber de astronomia, through which it became known in Europe, as did the armillary sphere [37].

Figure 3: North African universal astrolabe (probably from the 13th century) at the Museum of the History of Science, University of Oxford (Inventory n° 41122). This astrolabe uses the ‘universal lamina' described by Al-Zarqali, where a special form of rete rotates above a horizontal projection of the entire celestial sphere (Source).

A Jew from Montpellier in France translated it into Latin; King Alfonso of Castile made two translations of it into Romance (Spanish), whilst Regiomontanus in the 15th century published a collection of problems on the ‘noble instrument of the safiha' [38].

Al-Zarqali, most of all, is a genial astronomer, whose accomplishments are too many to tell here. Like other Muslim scholars he corrected the Greeks, which again, disproves the idea that Muslim science is a copy of the Greeks, for it both corrects such Greek science and adds discoveries which became the realm of the European West many centuries later. Ptolemy's exaggerated estimate of the length of the Mediterranean sea at 620 were first cut by al-Khwarizmi to 520, then by al-Zarqali to the near the correct value of 420[39]. Al-Zarqâli also wrote a treatise on the movement of the fixed stars, a discussion of theories regarding the solar year [40]. His astronomical observations were the best of his age, and enabled him to prove for the first time the motion of the solar apogee with reference to the stars [41]. Motion of the solar apogee with reference to the stars which he said amounted to 12.04' a year; and also gave a value of 770 50' for the longitude of the sun's apogee, and concluded that the inclination of the ecliptic oscillated between 23033' and 23053' [42].

In his Tratado relativo al-moviemento de las estrellas fijas, only preserved in a Hebrew copy, al-Zarqali sought to demonstrate mathematically the trepidation theory according to which the movement of the sphere of the fixed stars is determined by the movement of a straight line that joins the centre of the earth with a movable point on a base circle or epicycle [43]. Al-Zarqali explains the trepidation theory according to three models that situate the epicycle 1) in a meridian plane; 2) in the plane of the ecliptic, and 3) with two equal epicycles centred in the mean equinoctial points normal to the equator [44].

The construction of astronomical tables implied trigonometrical theories and computations which were generally explained in the introductory chapters to these tables. In Al-Zarqali can be found a section dealing with trigonometry, which includes tables of sines, cosines, versed sines, secants, and tangeants [45]. The work was translated into Latin by the Italian John of Pavia in 1154; William de St Cloud in 1296; a Hebrew version by Jacob Ibn Tibbon in 1301; and also translated into Portuguese, Catalan, Castilian, etc [46].

Al-Zarqali, however, is even more famed, and impacted for centuries on the Christian West with regard to his Toledan Tables[47]. The tables include the determination of the right ascensions, and the equations of the sun and the moon and of the planets; parallax; eclipses, and the setting of the planets; theory of trepidation or accession and recession; etc [48]. His work was translated into Latin by Gerard of Cremona, and was very popular for more than two centuries [49]. All subsequent tables for different locations in Europe were based on al-Zarqali's measurements. The tables of Marseilles (based on Al-Zarqali's Toledan Tables) were also adapted to the meridians of London, Paris and Pisa [50].

Raymond of Marseilles is one of the first who adapted Al-Zarqali's table to a European location, Marseilles [51]. Leopold of Austria, an Austrian astronomer and meteorologist, who flourished probably in the middle of the second half of the 13th century, composed an astronomical treatise which was professedly a compilation, entitled Compilatio de astronum scientia, divided into ten treatises, who also relied on al-Zarqali to great measure [52]. Alfonso the Wise's Tablas alfonsinas were also based on al-Zarqâli's work [53]. Robert of Chester's work was less a translation than an adaptation of the tables of al-Battani and al-Zarqali for the coordinates of London composed in 1149 [54].

Figure 4: Diagram of the movement of the sun (ecliptic).

Al-Zarqali, who was once given books at a late stage of his life to learn, later on had his own students such as the reputed Yahia Ibn Sayyid (d. 1144); and his influence was exerted upon some of the greatest astronomers of Islam, such as Ibn al-Kammad, Al-Bitruji, Abu' Hasan al-Murrakushi, Ibn al-Banna and others [55]. Just like Ibn Sina, al-Kindi, al-Farabi, Al-Razi, al-Farghani, and al-Khwarizmi, al-Zarqali was printed in Europe quite frequently, up to the sixteenth and even the 17th century [56].

Whilst Copernicus, for instance, relied profusely on al-Zarqali (and al-Battani) in his book De Revolutionibus[57], and on his treatise on the astrolabe [58], Abraham Zacut, whose solar declination tables were used by explorers of the 16th century to calculate latitudes, followed al-Zarqali on planetary and solar motion [59].